The present invention relates to a novel carbon-silicon carbide composite material and a method for the preparation thereof. More particularly, the invention relates to a carbon-silicon carbide composite body consisting of carbon as the matrix phase and silicon carbide as the dispersant phase dispersed in the matrix phase in an intimate state to serve as a reinforcing material so as to impart the composite material with greatly improved properties such as toughness, mechanical impact strength, resistance against heat shock, mechanical strengths, sliding characteristic, resistance against abrasion, resistance against erosion and so on as well as a method for the preparation thereof.
As is well known, shaped bodies of carbon are widely used as an engineering material by virtue of their excellent properties in many respects. One of the serious disadvantages of a shaped body based on carbon alone, however, is that the mechanical strength thereof is so low as not to be used as a constructing part. In this regard, many attempts and proposals have been made for developing a carbon-based composite material consisting of carbon as the matrix phase and a fibrous material, e.g., fibers and whiskers, as a reinforcing agent dispersed in the matrix phase. While such a carbon-based composite material is prepared usually by using a precursor of carbon such as a pitch or resin, to be carbonized by a heat treatment, a problem in such a carbon-based composite material is that the shaped body under the carbonization treatment exhibits great shrinkage and the resultant composite body is necessarily very porous.
A method has been proposed to solve the above mentioned problems in which a carbon-matrix composite material is formed with carbon fibers as the reinforcing agent and the composite body is subjected to a number of cycles each consisting of impregnation of the composite with a pitch and calcination of the pitch-impregnated composite so as to increase the density and improve other properties of the composite material. The application fields of this method, however, are limited because the procedure is very complicated and the method is not versatile in respect of the limited composite textures.
Another problem in carbon fiber-reinforced carbon-matrix composite materials is that, while the carbon fibers used as the reinforcing agent are usually in the form of long filaments, the composite material compounded with such long filaments is more or less anisotropic with limited applications so that it is desirable to use short fibers or whiskers in order to obtain an isotropic composite material. It is, however, not always a simple matter to obtain a fully uniform blend from a particulate or granular material for the matrix phase and a fibrous or acicular material for the reinforcing agent even in the use of short fibers or whiskers of carbon so that the mechanical strengths of the composite material cannot be as high as desired or expected. Even if uniformity could be ensured in blending of the matrix phase and the dispersed phase, the carbonaceous matrix material and the short fibers or whiskers are necessarily different in the behavior of thermal shrinkage in the course of calcination of the composite material which accordingly cannot be dense enough. Moreover, fibers embedded in the matrix phase of a composite material have a trend to be aligned, when the composite material is molded by compression, in the direction perpendicular to the direction of the compressive force not to give a fully isotropic shaped body of the composite material.
As is described above, carbon-based composite materials reinforced with a fibrous or acicular reinforcing agent prepared by the conventional method in the prior art have several problems and disadvantages. This is the reason for the very limited application fields of such composite materials despite the expected excellent and desirable properties thereof when the preparation process could be ideally performed. In addition, it is unavoidable in the preparation of conventional composite materials compounded with a fibrous or acicular reinforcing agent that the fibers or whiskers are more or less damaged to adversely affect the properties of the composite material.
The inventors have previously proposed (1) a method for the preparation of a composite material consisting of the matrix phase of carbon and the dispersed phase of particles of a ceramic material such as boron carbide and silicon carbide very uniformly dispersed in the matrix phase in Japanese Patent Publication No. 58-38386 and (2) an oxidation-resistant carbonaceous material prepared by the simultaneous compounding with particles of both boron carbide and silicon carbide to exhibit extremely high resistance against oxidation in Japanese Patent Publication No. 62-12191 and also discovered that a carbon-based composite having excellent resistance against oxidation and a large fracture energy can be obtained by compounding the above mentioned composite material further with short fibers of carbon.
Though with excellent oxidation resistance and a large fracture energy to be noteworthy for many applications, this last mentioned composite material has a problem due to the difference in the thermal expansion or shrinkage between the matrix phase and the dispersed phase that the denseness of the composite material cannot be high enough and the bulk density thereof after calcination is unavoidably low as compared with similar composite materials not compounded with short carbon fibers. The problems of damage in the fibrous material and low uniformity of dispersion thereof are of course not solved therein. Moreover, the high oxidation resistance of the composite material can be obtained only by the simultaneous compounding with silicon carbide particles so that the manufacturing process is necessarily complicated. As a possible alternative way, oxidation resistance could be imparted to the composite material by compounding with whiskers of silicon carbide in place of carbon fibers. However, silicon carbide whiskers have high rigidity as compared with carbon fibers so that silicon carbide whiskers are damaged more in the process blending than carbon fibers. It is reported, in Journal of the Japan Society for Composite Materials, volume 9, No. 3 (1983), that a composite material prepared from a graphite powder, a thermosetting resin as a binder and 2.5% by weight of acicular silicon carbide particles has a greatly improved abrasion resistance but the above mentioned problems are not solved therein.